MRI systems utilize gradient amplifiers for driving a gradient coil, which generates gradient fields. Such gradient fields are magnetic fields having a strength which varies linearly in a given co-ordinate direction in order to define the location for imaging to be formed by the MRI system by addition of this gradient field to a strong steady, uniform field. To this end, current pulses of an intensity of the order of magnitude of more than 600 A at a voltage of the order of magnitude of more than 1500 V are applied to the gradient coils, the rise time of the pulses being of the order of magnitude of 0.2 ms whereas the pulse duration is in the range of about 1 ms to 10 ms.
The magnetic field characteristic of the gradient coil, with respect to time, is generally of a trapezoidal shape, with very steeply sloping sides. For this reason, the current provided by the gradient amplifier to drive the gradient coil is desired to have a trapezoidal-shaped characteristic. There is a tendency towards shorter rise times with larger maximum currents so as to reduce the time required for the acquisition of MRI information for the formation of an MRI image; this offers advantages inter alia in respect of image sharpness and also in respect of imaging of moving objects. However, as the gradient coils exhibit an inductive behavior to the gradient amplifier, a higher voltage is required so as to achieve a shorter rise time of the pulses. Increasing the currents and the voltages to be supplied by the gradient amplifier, in combination with a shorter rise time, gives rise to problems concerning the electronic components in the gradient amplifier. Losses in the semiconductor components cause a significant development of heat, giving rise to cooling problems. These problems can be mitigated partly by using a switched inverter of the multilevel type (multilevel inverter).
One of the prior arts suggests using a HDx gradient amplifier topology for a high fidelity driver which is typical in the magnetic resonance imaging system. One limitation associated with such a topology is that the gradient amplifier comprises twelve switches and four capacitors at DC side making the gradient amplifier bulky and difficult to control.
Hence there exists a need for a simple, compact, efficient and reliable multilevel inverter that can be employed in the gradient amplifier of the MRI system.